Built: 1604 to 1605 and 1604 to 1626
Builders: Elector Johann Adam von Bicken and Elector Johann Schweickhardt von Kronberg (1604-1626)
Style: Renaissance

The building consists of plastered quarry stone masonry. The high tiled gable roofs and the schematically arranged stone post windows with sandstone frames give the impression of a princely castle complex.

On the outside of the north and west wings are long rectangular window slits. They still refer to the building's function as a weapons storage facility. A special feature of the building are the three-story, curved rollwork gables erected on the western main wing. The individual gables are visually divided evenly by the cornice. They are the oldest and most ornate of their kind in Mainz.

On the northern gable side of the main wing, at the main entrance to the armory, there is a replica of a mighty Renaissance portal. In the inner courtyard of the armory stands an octagonal stair tower. It extends to the attic and is topped with a so-called Welsh dome in the shape of an onion. Due to the greater depth of the south wing compared to the north wing, it appears slightly offset. The open fourth side of the three-wing complex is closed off by the newer armory in front.

Today, the Rhineland-Palatinate State Chancellery is located in the Old Armoury.
The elaborate decorative elements on the façade and the well-designed layout – although greatly altered from the original building – make this Renaissance building particularly worth seeing.